1. Field of the Invention
The invention relates to a surface acoustic wave filter, and particularly to a surface acoustic wave filter which is used in a high-frequency region.
2. Related art of the Invention
Recently, studies on a surface acoustic wave device which can be used in a filter have been intensively conducted. In order to cope with recent development of mobile communications and increase of the frequency, particularly, development of a surface acoustic wave device has been vigorously made.
Conventionally, several kinds of filters which are configured by a surface acoustic wave device and which are used in the high-frequency band, particularly in the barnd of the hundreds of megahertz are known. Typical examples of such filters are as follows: a filter which is of the so-called ladder type and configured by plural surface acoustic wave resonators, such as that disclosed in Japanese patent publication (Kokai) No. SHO52-19044; a filter which is of the so-called multielectrode type, such as that disclosed in Japanese patent publication (Kokai) No. SHO58-154917; and a filter which is of the so-called longitudinal mode type and in which surface acoustic wave resonators are juxtaposed and coupling among the resonators are used, such as that disclosed in Japanese patent publications (Kokai) Nos. HEI32-222512, SHO61-230419, and HEI1-231417. Most of these filters handle an unbalanced signal and are set to have a characteristic impedance of 50 ohms in order to meet user requests.
Recently, there is a movement afoot to balance a high-frequency circuit in order to improve the performance of the circuit. In a high-frequency circuit of a portable phone or the like, for example, an unbalanced circuit is used in the high-frequency side and a balanced circuit in the low-frequency side. When a circuit is balanced, merits can be produced that the resistance to noise is improved and that the circuit can be driven at a lower voltage. However, such a balanced configuration requires circuit parts to be ready for a balanced circuit. In an unbalanced type circuit of the prior art, parts have an input/output impedance of 50 ohms. When a circuit is balanced, however, the impedance is not always 50 ohms. In a transitional period from an unbalanced circuit to a balanced circuit, particularly, parts are necessary which have unbalanced terminals at the input and balanced terminals at the output. An example of such parts is a balun. A balun has a configuration which can control the impedances of the balanced and unbalanced terminals. When a circuit of the prior art, i.e., an unbalanced circuit is to be used, however, a balun is not necessary. When a balun is used for deforming a circuit of the prior art into a balanced circuit, therefore, defects such as the increased cost of parts for the balun and the increased area required for mounting the balun are produced.